1. Field of the Invention
The invention is related to methods and apparatus for diagnosing sleep disorders. More particularly, the present invention is directed to ambulatory diagnostic apparatus and methods for use in treating sleep apnea.
2. Discussion of Related Art
A coincidence of a reduction in active changes of electrical heart and respiration potentials for a substantial length of time, followed by an increase in heart rate, is known to be an indication of sleep apnea. However, this type of record is subject to electrical potential artifacts that compromise the usefulness of these signals, as noted in U.S. Pat. Nos. 4,422,458 and 4,580,575. Also, snoring itself may be a risk factor when obstructive sleep apnea is suspected, depending on its pattern, and such devices cannot evaluate this condition. Also different types of sleep apnea, such as obstructive apnea and centrally caused apnea, are amenable to different therapeutic measures. These devices cannot provide the differential diagnosis needed for treatment of an apnea patient's problem.
The use of acoustic screening of respiration by a behavior modification device that detects loud snoring, or an alarm device that wakes the patient when a period of silence occurs that indicates a dangerously long acute sleep apnea episode, is disclosed in U.S. Pat. No. 4,715,367. Other acoustic alarm devices are disclosed in U.S. Pat. Nos. 4,306,567 and 4,129,125.
Snoring is generally known to be an indicator of obstructive apnea, as distinct from neurological, centrally-caused apnea. However, the occurrence of snoring and silence, either in combination or separately, does not provide sufficient information to provide a diagnosis that is adequate for the treatment of sleep apnea. For example, some brief, non-acute sleep apnea episodes disturb sleep, and can produce symptoms such as fatigue and irritability that are cumulative and can become clinically significant. However, similar silences may be detected that are not indicative of sleep apnea--because of the acoustic difference between thoracic and abdominal breathing.
Prompt, accurate diagnosis of sleep apnea is also important because sleep apnea is a condition that can be treated, and even corrected, if it is detected and the nature of the problem causing it is identified early enough. Also, sleep apnea episodes that are not themselves life threatening are, nonetheless, a serious risk factor for the survival of cardiac patients. Early, accurate diagnosis and prompt treatment of sleep apnea is particularly important for these high-risk apnea patients.
Thus, when sleep apnea is suspected, the patient is commonly referred to a hospital "sleep center," which can provide a complete diagnosis of the patient's condition during sleep, based on a detailed, polysomnographic record. However, polysomnographic evaluation--which monitors thoracic air pressure, partial (P) and saturated (Sa) blood oxygen pressure, abdominal and thoracic respiration potentials, breath sounds, heart rate, and possibly other indicators as well--must be done on an in-patient basis.
Of course, such a hospital stay is expensive. It is also disruptive of the patient's daily routine in a way that may, in itself, produce sleep disturbances. Such additional disturbances may generate misleading results, and could possibly interfere with diagnosis of the underlying cause of a patient's clinical symptoms. Furthermore, the manual review process required to evaluate the voluminous, detailed record thus produced is both highly technical and very time-consuming.
It has been surprisingly found that the automated report provided in accordance with the present invention has produced no false negative indications despite its much greater simplicity and compactness, when that report is evaluated by skilled medical personnel. The invention also, surprisingly, has provided a complete diagnosis for 54% of the patients tested. For the remainder, the invention permits medical personnel to limit the number of patients referred for full polysomnographic evaluation to cases where centrally-caused apnea is indicated.